JAKARTA (Reuters) - Indonesian authorities said on Monday several suspected Islamist radicals arrested last week on suspicion of plotting to attack neighboring Singapore were part of a group that has "dozens of members" and has been active for two years.

Counter-terrorism forces arrested six men on Friday on Batam island, just to the south of Singapore. The arrests comes amid mounting concern about the spread of Islamist ideology in Indonesia, the world's largest Muslim-majority nation.

National police spokesman Boy Rafli Amar said the group mostly recruited members online and was taking instructions from Bahrun Naim, an Indonesian who has joined Islamic State (IS) in Syria.

"The GRD group has been in operation for two years and has dozens of members," Amar told a news conference in Jakarta, referring to the group named after its leader Gigih Rahmat Dewa.

"This group is an extension of Bahrum Naim's vision and mission. We cannot underestimate them."

Authorities in Indonesia say dozens of men have gone to join IS in the Middle East. Security officials fear that IS leaders are now asking supporters in Southeast Asia to launch attacks at home.

Counter-terrorism officers rounded up the six men after tracking them for months on social media. One of the suspects was later released, Amar said on Monday.

Dewa, 31, and others in his group are believed to have been in direct contact with Naim via Facebook.

Police say Naim masterminded an attack in the capital, Jakarta, early this year that killed four people, and has since continued to instruct people online on how to mount attacks and assemble bombs and weapons.

Authorities say they are working to block radical online content.

Following the arrests on Friday, Singapore's home affairs minister, K. Shanmugam, said the men had planned a rocket attack on Marina Bay, a central entertainment area with a waterfront promenade, giant ferris wheel and a swanky casino resort.

Police seized bomb-making materials, guns and arrows from suspects' homes in Batam. But, police said, there were no signs that a rocket attack was anything more than a plan.

"There was no specific time frame for them to attack Singapore, but that was their target," Amar said.

The vast majority of Indonesians practice a moderate form of Islam. The country saw a spate of attacks from homegrown militants in the 2000s, including a bombing at a nightclub on the holiday island of Bali that killed 202 people.

(Additional reporting by Glenys Kirana; Writing by Kanupriya Kapoor; Editing by John Chalmers, Robert Birsel)